Read more about the Illinois woman's disappearance on CNN affiliate WGN-TV in Chicago.

(CNN) -- An Illinois woman walked away from a car crash that killed her husband, authorities believe, but they have found no trace of her other than footprints in the snow leading to a road.

A search was in its third day Tuesday for Tanya Shannon, 40, of Ransom, Illinois, according to the LaSalle County, Illinois, Sheriff's Office.

Ground operations began at 9 a.m. with air operations starting at noon, Sheriff Tom Templeton said. "They're looking," he told CNN sister network HLN. "They're expanding their search area just a little."

Shannon was last seen along with her husband Dale Shannon, 41, at a Christmas party Saturday night in Streator, about 17 miles west of Ransom, relatives told CNN affiliate WGN and the Chicago Tribune.

"They were dancing together, really cutting up a rug," Dale Shannon's sister Donna Baker, who was at the party, told the Tribune. She said the couple -- married 20 years with four daughters ages 4 to 15 -- left the party arm in arm.

Her brother was sober, she told the newspaper, and was driving his wife's car.

About 1:45 a.m. Sunday, a deputy on patrol in a rural area of Brookfield Township found a one-car traffic accident, the sheriff's office said in a statement. "The deputy found a male occupant of the vehicle in the driver's seat deceased," the statement said. "Evidence at the scene indicated that a second person was also in the vehicle at the time of the accident. Further investigation revealed that the second person was possibly the wife of the deceased driver."

The car slid backwards into a pole, Templeton said, with enough force to break the backs of both the front seats. A relative told the Tribune Dale Shannon's back was broken.

Footprints were reportedly found leading from the car to a nearby road. But there, Tanya Shannon's trail ends.

"There's nothing for us to believe there's any issues other than exactly what happened, a traffic accident that claimed her husband's life, that she was at least able for a period of time to walk away from and move up to the roadway," the sheriff said. "And once she was on the roadway, that's where we lost track of her."

He said he couldn't speculate on whether or how badly Tanya Shannon was injured in the crash. "She was at least able to get up to the road," he said. Police have her cell phone, he said.

Police have not ruled out the possibility that Tanya Shannon was picked up by someone. But "if she was able to walk and able to move, how come she hasn't contacted any of her family?" Templeton told WGN.

Tanya Shannon was last seen wearing a red dress -- described by the Tribune as a ball gown -- and a gray fleece hooded jacket, police said. The dress should have made her easy to spot in the rural, snowy landscape.

"We were told from the family she was an extremely loving mother," Templeton told the Tribune. For her to just leave with no warning "would be incredibly out of character," Templeton said.

"To be out in the elements as they were that night, dressed only as she was, it's very doubtful that you can survive terribly long."

The crash site was remote, Templeton told the newspaper. A nuclear power plant lies to the north, but the area is surrounded by farmland.

"It's crazy," Baker told WGN. "We're holding on to every ounce of hope we have, just to hope for (a) safe return. ... It's been like a nightmare you can't wake up from. If anybody has anything, seen anything, please, please contact us, because these four girls need something."

Search and rescue operations have so far involved dogs and aircraft from the Illinois State Police, along with more than 50 searchers, according to WGN and the sheriff's office.

Tanya Shannon is described as being 5 feet 2 inches tall and weighing 125 pounds, with shoulder-length strawberry-blond hair and green eyes, according to the sheriff's statement. She also wears glasses.

It's very mysterious what might have happened to Tanya," Baker told the Tribune. "We checked the area hospitals and nothing, so we don't know if someone picked her up and took her somewhere. We just don't know."

Templeton said he's never seen a case like this in his 12 years as sheriff. "Anything that's even remotely close has been gone over several times," he told the newspaper.

Anyone with information regarding Tanya Shannon's whereabouts is asked to contact the LaSalle County Sheriff's Office.